Muscle Length Modulates Recurrent Inhibition and Presynaptic Inhibition of Ia Afferents Differently Depending on Type of Contraction

It is well documented that, in soleus, motoneuron output and the effectiveness of activated Ia afferents to discharge α-motoneurons both decrease during eccentric contractions. Evidence suggests that these regulations can be explained by recurrent inhibition and greater presynaptic inhibition of Ia afferents. However, the influence of muscle length on the regulation of the effectiveness of activated Ia afferents to discharge α-motoneurons observed during eccentric contractions remains unclear. We conducted a study on 16 healthy young individuals. We used simple and conditioned Hoffmann reflex with different conditioning techniques such as H' method, D1 method and heteronymous Ia facilitation coupled with electromyography during eccentric, isometric and concentric contractions at long, intermediate and short soleus muscle lengths. Our results confirm that during eccentric contraction effectiveness of activated Ia afferents to discharge α-motoneurons decreases only at intermediate and short muscle lengths but is similar between all contraction types at long muscle length. Findings are similar for recurrent inhibition. Presynaptic inhibition of Ia afferents is significantly more pronounced during eccentric contractions compared with isometric and concentric contractions at long muscle length. These findings highlight the significant impact of joint position on spinal regulatory mechanisms, particularly during eccentric contractions. Our analysis also reveals that recurrent inhibition and presynaptic inhibition of Ia afferents are more pronounced at long muscle lengths compared to short muscle lengths, regardless of the contraction type. This result has uncovered a previously unknown function of recurrent inhibition, potentially modulating motoneuron frequency discharge in response to changes in muscle length.